Ion mobility spectrometry is a versatile technique used to detect the presence of molecular species in a gas sample. The technique has particular application in detection of explosives, drugs, and chemical agents in a sample, although it is not limited to these applications. Portable detectors are commonly used for security screening, and in the defense industry. However, conventional portable devices are still nonetheless relatively large.
Ion mobility spectrometry relies on the differential movement of different ion species through an electric field to a detector. By appropriate selection of the parameters of the electric field, ions having differing properties will reach the detector at differing times, if at all. Time of flight (TOF) ion mobility spectrometry measures the time taken by ions when subject to an electric field to travel along a drift tube to a detector against a drift gas flow. By varying the electric field, ions of different characteristics will reach the detector at different times, and the composition of a sample can be analyzed. This form of spectrometry relies on the length of the drift tube for its resolution; the longer the drift tube, the more powerful the detector. This restricts the possible miniaturization of such spectrometers, since there is a limit to the lower size of the drift tube which may effectively be used. Further, given that relatively high electric field strengths are necessary, the restriction on drift tube length also results in the need to use relatively high voltages in the device, which may be potentially hazardous to the operator and further restricts the possibility of miniaturization of the device.
A variation on TOF ion mobility spectrometry is described in U.S. Pat. No. 5,789,745, makes use of a moving electrical potential to move ions against a drift gas flow towards a detector. A plurality of spaced electrodes are alternately pulsed to generate a moving potential well, which carries selected ions along with it. This device is unsuited to miniaturization due to, among other reasons, the need for a pump to produce the drift gas flow.
Field asymmetric ion mobility spectrometry (FAIMS) is a derivative of time of flight ion mobility spectrometry (TOFIMS), which potentially offers a smaller form factor. Existing designs, however, use moving gas flows and high voltages, which are undesirable for microchip implementations. Scaling is further hindered by molecular diffusion, an effect that becomes significant in the micron regime. Background information relating to FAIMS can be found in L. A. Buryakov et al. Int. J. Mass. Spectrom. Ion Process. 128 (1993) 143; and E. V. Krylov et al. Int. J. Mass. Spectrom. Ion Process. 225 (2003) 39-51; hereby incorporated by reference.
Conventional FAIMS operates by drawing air at atmospheric pressure into a reaction region where the constituents of the sample are ionized. Chemical agents in vapor-phase compounds form ion clusters when they are exposed to their parent ions. The mobility of the ion clusters is mainly a function of shape and weight. The ions are blown between two metal electrodes, one with a low-voltage DC bias and the other with a periodic high-voltage pulse waveform, to a detector plate where they collide and a current is registered. Ions are quickly driven toward one electrode during the pulse phase and slowly driven toward the opposite electrode between pulses. Some ions impact an electrode before reaching the detector plate; other ions with the appropriate differential mobility reach the end, making this device a sort of differential mobility ion filter. A plot of the current generated versus DC bias provides a characteristic differential ion mobility spectrum. The intensity of the peaks in the spectrum, which corresponds to the amount of charge, indicates the relative concentration of the agent.
While this arrangement offers the possibility for greater miniaturization than conventional TOFIMS, the need to generate a gas flow requires the presence of a pump (diaphragm or similar) which using present technology limits the lower size of such a device. Representative examples of such devices are described in U.S. Pat. No. 6,495,823 and U.S. Pat. No. 6,512,224.
It would be of benefit to provide miniaturized ion mobility spectrometers for use in sensing techniques; not only would these be suitable for covert use or for large scale distribution, the smaller size will allow use of lower voltages in the device. Devices with no or fewer moving parts than conventional devices would also be of benefit, in that they would be more robust than conventional sensors, and so suitable for deployment in high-traffic areas or in harsh environments.